This invention pertains to dual conduit drill stem members.
In the co-pending application of William W. Bishop entitled Dual Conduit Drill Stem Member and Connections filed concurrently herewith, the disclosure of which is incorporated herein by reference, there is described a dual conduit drill stem member upon which the present invention is an improvement. As there disclosed a dual conduit drill stem member comprises a tube radially positioned within a steel pipe by spacers axially and azimuthally spaced along the length of the tube. The ends of the tube are welded to the box and pin connectors which are provided with dual compression seals, and the ends of the pipe are provided with threaded pin and box tool joint members, for making connection with the tube and pipe of other drill stem members. The tube connectors are provided with radial fins welded thereto, the fins on each connector engaging an outwardly facing shoulder in the adjacent tool joint member and being shrink fitted within the tool joint member.
The shoulder and shrink fit engagement of each tube connector to the adjacent tool joint member is advantageous to the compression seal construction in that as the tool joint members are screwed together the tube connector seals are compressed. If the tube connectors were free to move axially relative to the tool joints, make up of the tool joints on the pipe would not effect compression of the tube connector seals, and other means, for example the weight of the tube, would have to be used to effect the desired seal compression. If only one of the tube connectors were fixed axially, the other being free, it would be possible to effect seal compression upon make up of the tool joints on the pipe by transmission of the compressive load through the tube from the free connector to the fixed connector, but the degree of compression would then vary as the pipe bent, elongated or contracted relative to the tube. Therefore, it is preferable to rigidly anchor both tube connectors to their respective tool joint members as in the construction of the aforementioned Bishop application. However, some problems may be created by anchoring both the connector to the respective tool joint members as will be explained hereinafter.
The drill stem member construction disclosed in the aforesaid Bishop application is eminently satisfactory for certain applications, such a drilling blast holes in the process of strip mining. In such use the overall length of the drill stem, which comprises several drill stem members connected together, is usually less than 100 feet. The weight of the drill stem is insufficient to properly load the drill bit and the drill stem is pushed downward by applying a portion of the weight of the drill rig to the drill stem through the swivel connection. The drill stem is therefore all operated in compression. Furthermore, the hole and bit diameter is not much larger than the pipe's tool joint diameter so that the sides of the drill stem may be supported by the hole. This is also the case in the drilling of small capacity water walls, the depth and diameter of which are small.
More severe operating conditions are encountered when deeper and larger diameter holes are drilled, as in the case of deep water wells, irrigation wells, mine shafts, underground storage chambers, and the like. A principal problem encountered is that of excessive vibration of the tube, due in large part, it is believed, to the effect of high pressure fluid passing through the drill stem. For example, the fluid may be air flowing down the annulus between pipe and tube to a level below the level of water standing in the hole being drilled and air and water flowing up the tube above that level. The vibration due to movement of such an air water mixture may be likened to the water hammer in a heating system. Other vibrations originate from the reaction of the drill bit at the bottom of the hole. Vibration, whether it be axial, torsional, or transverse, imposes excessive stress on all parts of the drill stem which may result in failure.
In the case of deep holes, the uppermost lengths of drill stem may be in tension, adequate bit weight being provided by the lower part of the drill stem, which may include drill collars. Tensioning of the drill stem places the pipe in tension and this tension is transmitted through the connection of the tool joint members and tube connectors to the tube. While this is advantageous in that the tube shares the load with the pipe, the stress due to tube tension when added to that due to vibration may cause failure. Also, the tensioning of the tube may itself increase the vibration thereof like the tensioning of the string on a musical instrument. The tube may therefore vibrate with considerable amplitude, both torsionally and transversely.
In the case of big holes, the drill stem may whip around in the hole as the pipe rotates, causing considerable bending of the pipe. In any bending situation some of the fibers are in tension and some of the fibers are in compression. Such bending stresses reverse as the pipe rotates and the direction of bending changes. These reversing bending stresses are imposed on the weld structures of the drill stem.
Both in the case of vibration and reversing bending stresses, the weld structures of the drill stem may fail due to fatique. The present improvement is directed to reduction of such failure.